In U.S. Pat. No. 4,294,656, by U. Beck et al., a suspension is exposed to a field of ultrasonics. A voltage or potential is then measured in the field with a pair of electrodes which are separated by a distance that is about an odd half multiple of the ultrasonic wavelength in the suspension. The measured potential can be regarded as a vibration potential that can be used to control for example the retention or addition of flocculating agents to the liquid suspension. A further description of the technique can be found in an article entitle: "Measuring Zeta Potential by Ultrasonic Waves", by U. Beck, R. Zana and E. Rohloff and published on September 1978 at pages 63-65 of a publication entitled Tappi, this publication being also identified in U.S. Pat. No. 4,294,656.